1. Field of the Invention
The present invention relates to a battery vehicle, and more particularly, to a battery vehicle in which wheels are independently driven by separate inverters and motors.
2. Description of the Background Art
In battery vehicles such as a battery folk lift, a drive system may be adopted in which two right and left wheels are independently driven by separate inverters and motors. Such a driving system is effective for simplifying a power transmission mechanism and improving driving efficiency.
It is important for general systems including two or more motors that these motors are cooperatively controlled to improve energy efficiency as a whole. A technique of cooperatively controlling a plurality of motors is disclosed in, for example, Japanese Laid Open Patent Application (JP-A-Heisei 6-326908). According to this conventional technique, optimum current distribution to the plurality of motors is achieved by setting priority levels to a plurality of motors when a driving current is required from the motors.
An important thing in a driving system using a battery as a power source is to make efficient use of the battery. For this purpose, an inverter is preferably controlled to output a closest output voltage to a battery voltage. However, this control sometimes causes problems when variation arises in characteristics of two motors.
Firstly, when there is variation in characteristics of the motors, there is a case where an output voltage necessary for outputting a required torque is lower than the battery voltage in one motor while being higher than the battery voltage in the other motor. In such a case, to protect the latter motor and the inverter connected thereto, the inverter connected to the latter motor must be controlled to output an output voltage smaller than the output voltage necessary for outputting the required torque. However, torques of the right and left wheels does not become same through a simple control of the output voltage so that the battery vehicle moves in an undesirable direction.
Secondly, when regeneration of power to the battery is carried out, there is a case where a regeneration voltage supplied from one motor to the battery through the inverter may be smaller than an allowable maximum voltage while a regeneration voltage supplied from the other motor to the battery through the inverter may be larger than the allowable maximum voltage. In such a case, in order to protect the battery, it is necessary to carry out a weak field control of the latter motor to decrease the regeneration voltage. However, when the field weakening control of the latter motor is carried out, the torques of the right and left wheels are not same so that the battery vehicle moves in an undesirable direction. If a battery having a sufficiently high battery voltage is used in consideration of variation in characteristics of the two motors, the above problems may be avoided. However, the use of such a battery is problematic in terms of economical efficiency.
Therefore, a battery vehicle is demanded which can make best use of capabilities of a battery even when variation arises in characteristics of two motors driving the wheels.